The basic material needed is a digital force meter equipped with precision force gauges to retain the peak force applied on a digital display and with a grid or wire system that allows mouse grip by either or both paws. Data can be collected either manually (reading the values on the display) or on-line through a RS232 connection with a computer. A number of different force meters for the determination of grip and limb strength are commercially available (PanLab, Ugo Basile, Columbus Instruments, TSE Systems, etc). However they are not considered to be technically equivalent as there are major differences in design and electronics. A grip strength meter with an axial force transducer is to be preferred to one with a lever-type force transducer (the results of the later is easily inferred by the physical law of the lever).
• All commercially available force meters are digital dynamometers produced by different brands and re-sold for the specific animal use by biological research companies. These may re-sell it either directly or via local country specific representatives, with prices in the order of 1200-3000€ (for meter and grid/wire, based on direct or mediated sell, money exchange rates, taxes etc). Few commonly used models are listed below:
• Chatillon® DFE Series available with capacities from 2 lbf (1 kgf, 10 N) accuracy of better than 0.25% full scale; AMETEK TCI Division • Chatillon Force Measurement Systems;
Largo, Florida (USA); re-sold by Columbus Instrument, USA
• CENTOR Easy force meter, 25N maximal force (see figure below); CatNo: CNR EA 25;
produced by Andilog Technologies SA, Chaville, France; re-sold by PanLab and others
Grid or wire for the grip strength measurements (specifically designed grids (grids only!) are sold by PanLab for 350€, however can easily be assembled from materials which can be purchased from a local do-it-yourself store)
1) Custom made computer cable for RS232 connection (optional, for the direct transfer of data to a computer). The cable terminal diagram is the following:
Centor Cable Computer
SubD 15 pin, m Function RS232 (9 pin SubD, f)
3 RS232 RxD 2
4 RS232 TxD 3
13 Shield 5
shield Shield shield
2) A software (optional for the direct transfer of data to the computer; only in conjunction with the cable) such as HyperTerminal software for the PanLab meter (data transfer settings; 9600 Baud, 8 data bits, 1 stop bit, no parity, no flow control; as part of the Windows operating system the HyperTerminal program is free of charge). Generally, after acquisition and for further processing, the results can be saved as a text file or can be copy-pasted into any other application, i.e. Excel thus allowing easy management and statistical analysis of the original data. Using the optional cable and software, the data are transferred directly to the computer, along with a date and time stamp. This ensures the best possible traceability and avoids possible writing or typing errors.
Screenshot of the data as can be seen, if the data are transferred from the Centor Easy force meter to the HyperTerminal program. Similar data sheet can be obtained with other software for data acquisition.
Special comments:
The proper functioning of the force meter can be verified by exerting a defined maximal force on it as follows:
a) affix a paper clip to the grip bar (aim: paper clips are magnetic, the grip bar is usually made of stainless steel and is thus not magnetic)
b) attach a magnet to the paper clip in a defined manner (ideally the contact to the magnet should be just on one edge of the paper clip (not on the side) and the place of contact on the magnet should be also defined: this is easier if you use a small magnet).
c) with a slow, constantly increasing force, pull the magnet until the paper clip is released. If the contact between the paper clip and the magnet is sufficiently defined, the force necessary to break the contact will be relatively constant.
First, perform the check with the magnet in the middle area of the bar: constant values with a data spread of <2% should result. A larger spread of the data will indicate that the electronics are not functioning appropriately (the electronics of some grip strength meters are inappropriate, i.e. Ugo Basile). If the first test was successful, then repeat the same test first on the left and then again on the right side of the bar: the results should be the same, independently of which side of the bar the measurement was made. A difference in the measurement values from the left and right side indicate that there might be a lever-effect:
some of the marketed grip strength meters are error prone due to the selection of a force transducer where the results are dependent on the animal’s holding position on the bar.
Therefore, such force transducers (e.g. the one from Ugo Basile) are not reliable. It is sufficient to perform these tests just once, on purchase and installation of the device.
The diameter of the pulling bar is also of particular importance. A very thin pull bar (as that of Ugo Basile) may allow a very tight grasp that requires a very strong pull to break it. In addition, the use of a very thin bar or wire could result in an animal wounding its paws. A very thick bar could result in weak grip strength values. A suitable bar should be 1-2 mm in diameter and composed of non-flexible metal, allowing an efficient grasp that can easily be broken by the operator.
The force meter has to be fixed to a firm base (so to be stable during the pull and not move as a result of mouse grasp), at a defined height above the table or surface. Positioning the force meter on the edge of a desk or table-top is a good idea, as upon breaking the grasp, the mice are then not likely to hit the top of the table.
The force meter has to be configured for the recording of the maximal force according to the manual and in the units (lb, kg, N) preferred by the experimenter.